Emerging environmental trends aimed at reducing greenhouse gas emissions, use of fossil carbon derivatives, and solid waste accumulation have created a growing desire to replace petroleum-based polyesters such as poly(ethylene terephthalate) (“PET”) with renewably-sourced materials and modifiers, particularly for applications such as packaging. Current commercial offerings include, for example, poly(trimethylene terephthalate), polylactic acid, polyhydroxyalkanoates, starch, and poly(butylene terephthalate-co-adipate). However, these products generally have inadequate mechanical properties, such as tear strength, for applications such as packaging. In addition, melt viscosity and melt strength are often too low for processing methods used in manufacturing packaging materials, such as melt-extrusion, film-blowing, and blow-molding.
Starch-containing compositions have been used to provide biodegradable resin compositions useful for manufacture of shaped articles such as rigid sheets, flexible films, or molded articles. For example, U.S. Pat. No. 5,043,196 discloses compositions that contain starch compounded into various water soluble polymers. “Thermoplastic starch” compositions using biodegradable polymers as melting aids have been disclosed in U.S. Pat. No. 6,235,815. Blends of starch with copolyesters (e.g., “3GTX” made from monomers comprising 1,3-propanediol (“3G”), terephthalic acid (“T”), and a linear aliphatic dicarboxylic acid (“X”)) can exhibit low bubble stability in film-blowing because of the low melt strength of the copolyester. Also, incomplete dispersion of the starch can lead to inadequate mechanical properties.
The capability to make a compostable polyester copolymer composition from one or more bio-sourced monomers that has adequate properties for packaging applications could significantly reduce the environmental footprint (i.e., non-renewable energy consumption and greenhouse gas emissions) of plastic bags. The present invention is directed to providing bio-compostable, and preferably bio-sourced, polyester copolymer blends with improved melt strength.